Abstract Porphyrias are genetic disorders caused by mutations in enzymes involved in eight sequential biosynthetic conversions that combine glycine and succinyl coenzyme-A to generate heme. Porphyrin accumulation can result in `secondary porphyrias' in association with other diseases such as hepatitis C virus infection. Current major unmet needs with regard to the porphyria disorders are our present limited understanding of the molecular triggers of porphyria acute attacks, aside from indirect association with fever or drugs; the reasons why some individuals develop significant end-organ complications such as the need for liver or bone marrow transplantation while others do not; and importantly the limited availability of drugs to treat the different porphyrias. Our central hypothesis is that proteotoxic porphyrin-mediated damage, coupled with modifier effects, contributes to the cellular damage and morbidity that is associated with porphyrias. This hypothesis will be tested by pursuing three specific aims: (i) Define the mechanism of porphyria-induced protein aggregation; (ii) Characterize potential modifiers that promote or protect from porphyria-induced injury in cell culture and animal models; and (iii) Use zebrafish porphyria models to screen and characterize small- molecule compounds that prevent tissue porphyrin accumulation. We have assembled extensive preliminary results to support the likely success of our aims, including substantial evidence for porphyrin-mediated protein aggregation that is dependent on the mode of porphyrin accumulation, evidence for fatty acid binding protein- 1 as a modulator of porphyrin-mediated cell and liver injury, and two novel acute zebrafish porphyria models we generated that are amenable to high-throughput drug screening with potential positive compounds that we propose to validate and characterize. Completion of our proposed aims should provide fundamental knowledge regarding how porphyrins cause protein aggregation, the subcellular compartments that are involved, whether such aggregation is due to direct covalent porphyrin-protein adduct formation, and the mechanism of aggregate turnover. We also anticipate being able to characterize specific porphyrin-binding proteins that regulate the propensity to accumulate porphyrins in cells and tissues. This could provide potential clues as to why some individuals with porphyria develop end-stage liver disease in the absence of concomitant underlying liver disorders, while other patients are spared serious liver-related complications. Importantly, we anticipate that our proposal will allow us to use two powerful novel acute porphyria zebrafish models we have developed to identify potential compounds that may ultimately have therapeutic utility. This proposal uses state-of-the-art technologies, multiple biochemical and animal model tools including zebrafish and mice, and introduces the novel concept of proteotoxicity as an alternative mechanism for porphyria exacerbations and progression.